The present invention relates to a television IF (intermediate frequency) stage, and more particulary, to such a stage that has a sound detector suitable for use with stereo sound.
TV audio may be detected by first forming an intercarrier signal (at 4.5 MHz in the NTSC system) by mixing in a synchronous detector the picture carrier and the sound carrier. Specifically, in this method the picture and sound carriers are amplified, after the tuner, in a common IF gain block. An IF filter attenuates the sound carrier more than the picture carrier. The picture carrier is then limited and bandpass filtered in an additional separate path to form a reference picture carrier. This reference picture carrier is mixed by the synchronous detector with the IF signal to form both the intercarrier sound signal and the baseband video signal. It is desirable to reduce the amount of video amplitude modulation (AM) of the reference carrier signal in order to minimize spurious second order effects in the video signal since it is only the input modulation which is the desired mixer output signal and hence the need for limiting and filtering the reference signal. In this type of single channel intercarrier sound system, any frequency modulation (FM) or phase modulation (PM) which is imparted onto the picture carrier during the transmission or reception process is not removed from the picture carrier since an equal amount of angle modulation is also imparted during the transmission or reception process onto the sound carrier. When the two carriers are mixed to form the intercarrier sound signal, this common mode interference is cancelled. However, phase shifts in the non-common mode path, i.e., the filtering and limiting path for the reference video carrier, are not cancelled. Thus, a buzz in the audio signal can result, especially when stereo sound is to be produced, which requires a high bandwidth audio channel in order to transmit the stereo difference and subcarrier signals.
Another type of IF sound detection system is called "quasi-parallel" IF, in which sound and video signals are separately demodulated in different channels. The sound carrier is not attenuated in the sound channel (as in the single channel system), resulting in higher sound sensitivity. In the sound channel, the picture carrier is mixed with the sound IF signal to form the 4.5 MHz intercarrier sound signal, the baseband video being superfluous in this channel. In an effort to minimize the amount of baseband video produced by the mixing process, it has been thought that the phase of the limited and filtered picture carrier signal should be shifted by 90.degree. so that this reference signal is in quadrature with the incoming picture carrier. In theory, this reduces the amount of AM that the limiter, in the subsequent FM detector, has to eliminate and therefore should eliminate buzz due to imperfect AM rejection in the FM detector.
However, the present applicants have discovered that most of the audio buzz which inteferes with multichannel audio is produced due to the filtering and limiting of the picture carrier so that the quadrature mixer has little effect on the higher frequency audio buzz. Therefore, the applicants have discovered that in order to minimize this audio buzz, great care must be taken in the IF circuitry, particularly in the picture reference path to prevent phase shifts. If the phase shift of the amplifier and limiters in this reference path vary as a function of signal amplitude (as a result of varying transistor or diode junction capacitances), the amplitude of the video carrier will modulate the phase of this reference signal. Similarly, if the rise and fall times of the limiter are not symmetrical, the zero crossing of audio signal will be modulated by the amplitude of the picture carrier--again resulting in PM. If the positive and negative clipping levels are not identical, there will be additional PM due to AM. Additionally, if the amplitude or phase of the bandpass filter in this reference path is not perfectly symmetrical, there will also be AM converted to PM. In general, any change in phase as a function of time will be demodulated by the FM sound detector as an instantaneous frequency deviation and as such will produce an impulse output during every luminance transition. Since luminance transitions tend to repeat for at least a few lines, these transitions cause considerble buzz at the horizontal scanning frequency and multiples thereof. Although no standard has as yet been selected for stereo TV transmission in the United States, since all the proposed systems use the horizontal frequency and multiples of it for pilot and carrier frequencies, these spurious buzz signals can cause interference with the detection of the stereo pilot signal, the detection of the second audio program, produce a whistle or buzz beat in the stereo signals, or produce a buzz beat in the second audio program depending upon the proposed stereo system.